This invention relates to a process for growing a gallium arsenide single crystal, and particularly to a modified Horizontal Bridgman Method in which no low temperature arsenic zone is provided.
Gallium arsenide (GaAs) is a direct band gap semiconductor and has a high radiative recombination efficiency. GaAs wafers have been used as substrates for a lot of optoelectronic devices such as infrared LEDs, solar cells, laser diodes and optoelectronic integrated circuits (OEIC). The requirements of most of the optoelectronic devices for GaAs substrates are low etch pit density and high doping concentration without microdefect.
Conventional boat growth methods for growing GaAs single crystals such as a two temperature zone horizontal Bridgman (2T-HB) method, gradient freeze (GF) method and three temperature zone horizontal Bridgmen (3T-HB) method are illustrated in FIG. 1. In the 2T-HB method, GaAs polycrystalline is introduced into a quartz boat which is placed in the high temperature zone of a quartz tube, and arsenic is placed in a low temperature zone of the quartz tube. The high temperature zone is kept at a temperature higher than the melting temperature Tf (1238 deg C.) of GaAs and the low temperature (T3) is kept within in a range of 610-617 deg C. After the GaAs is melted, it is cooled from the seed end to start the crystal growth by moving the furnace. The stoichiometry of the GaAs crystal is maintained by controlling the As zone temperature (T3) such that the vapor pressure of As equals to the dissociation pressure of the GaAs melt. In the gradient freeze method, the only difference is that the high temperature zone has a small slope from the seed end to the tail end and the crystal growth of GaAs takes place by the movement of the temperature gradient along the boat. Both the 2T-HB method and the gradient freeze method have the following disadvantages:
(1) The crystal is susceptible to dislocation generation due to thermal stress caused upon cooling from the high temperature zone to the low temperature zone without any intermediate annealing zone; and
(2) The GaAs crystal is liable to stick to the quartz boat and it is difficult to obtain a high purity single crystal because of the reasons described hereinbelow.
The GaAs melt and the quartz boat can induce the following reactions, EQU 4Ga(l)+SiO.sub.2 (s)=2Ga.sub.2 2zO(g)+Si(l) (1) EQU 3Ga.sub.2 O(g)+As.sub.4 (g)=2Ga.sub.2 O.sub.3 (s)+4GaAs(s) (2)
The reaction between the Ga and the quartz boat generates Ga.sub.2 O vapor and liberates Si into the GaAs melt. The Ga.sub.2 O vapor then diffuses away from the melt to the colder parts of the ampoule. At the colder part, Ga.sub.2 O vapor reacts with As.sub.4 vapor and condenses as Ga.sub.2 O.sub.3 and GaAs solids. This causes a deficiency in the Ga.sub.2 O.sub.3 vapor and makes the equation (1) continue to progress to the right-hand side. Thus, the more Ga reacts with the quartz boat, the more serious the sticking between GaAs ingot and quartz boat. Growth of the high purity GaAs is therefore impossible due to a large amount of Si incorporated into the GaAs melt.
The 2T-HB method is modified by the 3T-HB method in which an intermediate annealing zone (T2) of 1100-1200 deg C. is provided to alleviate the problem of thermal stress and a diffusion barrier is provided between the high temperature melting zone and the annealing zone. This diffusion barrier serves to retard the diffusion of Ga.sub.2 O vapor and thus a saturation vapor pressure of Ga.sub.2 O can be maintained in the high temperature zone. The reaction of equation (1) is nearly ceased after a saturation vapor pressure of Ga.sub.2 O being established and the sticking between the GaAs ingot and the quartz boat can be prevented. Although the 3T-HB method is more advantageous than the 2T-HB method and the GF method, it requires a relatively long furnace. For instance, for growing a GaAs single crystal of 50 cm, the furnace must be 3 meters long and the length of the ceramic supporting tube must be greater than 3.5 meter. Furthermore, a complicated furnace design is necessary to control the temperature changes in the 50 cm long As zone to within 1 deg C.